Each of these problem reports - and others - may come from people who don't read the instructions, for using the wizard. Alternately, some folks may be complaining about yet one more case of over done layered security.
The most obvious problems, in the blog creation process, come from people who don't understand how to use the "Create a blog" wizard.

Not every would be blog owner understands that 3 things must be done, to make the "Create blog!" button operative - and all 4 must be done, before the blog is actually created.

Besides the syntax issues when entering a blog name (URL), there is the unfortunate issue of competition in the creation process. Blog owners who are anxiously creating a new blog, based upon a current event - maybe a popular movie star, or an important political campaign - will be dismayed to see

Sorry, this blog address is not available.

If you are competing in real time, with other would be blog owners, for the name of your choice - and you take too long between Steps #1 - #3, and Step #4 - you may still see

Sorry, this blog address is not available.

Many people want to setup a blog, based on that blog name. Some may see the bad news, repeatedly, leading to one frequently seen complaint.